1. Field of the Invention
The field of the present invention relates to sound reproduction and, more specifically, to loudspeaker array configurations and related sound processing and systems.
2. Background
Sound reproduction systems incorporating speakers are commonplace in homes, theaters, automobiles, places of entertainment, and elsewhere. The number, size, quality, characteristics, and arrangement of speakers affect sound quality in virtually any listening environment. However, many environments have constraints which limit the number, size, or type of speakers which can be used, and their arrangement. These constraints may be technical, mechanical, or aesthetic in nature.
The listening experience involves not only the speaker but also its interaction with the listening room or area. The total sound arriving at the listener has two components—direct and reflected. The direct sound provides clues as to the perceived direction of the original sources, while the indirect component increases the apparent loudness, sense of spaciousness, and general ambience. These indirect effects are mostly positive in the sense that they tend to enhance the listening experience. But reflections can also color the sound, leading to unnatural changes in timbre.
Besides the potentially undesirable effects of indirect or reflected sound, the acoustic character of the listening room or area can be sufficiently obtrusive so as to mask the natural sound quality of the original source material. For example, at low frequencies, standing waves in a listening area may cause some low frequencies to be emphasized more than others, especially in domestic-sized living rooms. The resulting bass sound is often boomy and very position dependant.
Careful placement of the speakers can reduce some of the aforementioned problems, but typically provides a satisfying listening experience only within a limited area or “sweet spot.” Such a limited solution may have been acceptable in the past, when the audience for critical listening was limited to only one or two at a time. But this is seldom true today, when the sound source is usually multi-channel, more often than not accompanied by a picture, and with many more listeners seated informally over a much wider area.
With traditional stereo playback, the illusion of a continuous sound stage can be created between the two speakers when the listener is seated symmetrically in front of them. However, in practice, only those original sources positioned exactly in line with the speakers can be precisely located, while those between the speakers, often known as “phantom sources,” depend on the listener being equidistant from both. Any movement of the listener away from a position of symmetry will generally cause the phantom image to flop to the nearest speaker, thus destroying the stereo illusion.
The introduction of a center channel speaker, originally for film soundtracks, was intended to eliminate this effect and keep the dialog at stage centre. This it does, but often at the expense of narrowing the front soundstage. For this reason many critical listeners of multi-channel audio source material often prefer to omit the center speaker, even if it means a more restricted sweet spot.
Besides the potential problems caused by the shape or characteristics of the listening environment, additional difficulties may be presented by the nature of the speakers being used to reproduce the sound. Even well-designed speakers, having a smooth level axial frequency response, can often have an unnatural timbre or colored sound due to reflections from nearby surfaces. For this reason, the off-axis response as well as the direct sound from the speaker is desired to be both be smooth and well controlled. However, the problem of achieving well-controlled directivity in loudspeakers has proved difficult to solve. Speakers with some measure of directional control, such as horn-loaded speakers, have routinely been used in the cinema, although their continued use owes as much to their higher efficiency as it does to their ability to have a controlled directional response.
One downside to using horn speakers is that they distort at high levels due to the non-linearity of the air at the mouth of the horn. Additionally, their inability to maintain constant directivity over a wide frequency range typically requires the use of multiple horn speakers, each covering a different part of the audio spectrum. Low-frequency horns are extremely large and as a consequence are rarely used. For reproduction of the low frequencies it is common to employ more conventional direct radiator speaker systems.
Another well known type of speaker system with some measure of controlled directivity, also used for sound reinforcement purposes, is the so-called column speaker. A column speaker consists of a long line of closely spaced identical speaker drive units that can provide a degree of directionality in the vertical plane when placed upright. Longer lines permit greater directional control at lower frequencies, with the limit of directional control being generally set by relationship between the line length and the wavelength of sound at the lowest frequency of interest. The longer the line, the lower the frequency of beam control can be. The spacing between the drivers generally limits the upper frequency for control. A two-dimensional array, with both rows and columns of speaker drive units, is capable of providing control in all directions. The design of such systems is difficult, and its implementation is very expensive in general. Moreover, not until relatively recently has the processing power needed to provide good directional control over a wide frequency range been viable. The design of suitable transducers for inclusion in such an array is another matter altogether. Meeting the dual driver-design requirements of close spacing, for accurate high-frequency control, and the need for a large effective radiating area for good bass output are not inconsiderable. Existing speaker systems which purport to achieve some form of directional control use miniature drivers that are neither small enough for high-frequency control nor large enough for adequate bass output.
Coupled with the aforementioned challenges is the fact that, in many environments, it is desirable to minimize the visual impact of loudspeakers. One technique, for example, is to color or otherwise decorate the protective speaker faceplate to match the surrounding wall or object in which the drive unit in placed, or to hide the speakers behind an artificial painting. These types of solutions may not be satisfactory for all consumers, and may limit the possibilities for optimal speaker placement as well.
One technique that has been proposed for a type of speaker column having certain desirable characteristics relates so a so-called Constant Beamwidth Transducer (CBT) array. FIG. 1 is a diagram showing an oblique view of an example of a curved speaker line array 100, also known as a Constant Beamwidth Transducer (CBT) array, as known in the art. As illustrated in FIG. 1, the curved speaker line array 100, or CBT array, comprises a plurality of low frequency drive units 104 adjacent to a plurality of high frequency drive units 108. According to a particular technique, the CBT array 100 employs frequency-independent Legendre shading. This technique is described in more detail in, for example, D. B. Keele, Jr., “Practical Implementation of Constant Beamwidth Transducer (CBT) Loudspeaker Circular-Arc Line Arrays,” 115th Convention of the Audio Engineering Society, Paper 5863, October 2003, hereby incorporated by reference as if set forth fully herein. As explained therein, a CBT array is constructed using Legendre function shading of the transducer drive levels in order to maintain what has been described as frequency-invariant pattern control. Each transducer in the array is driven with a different signal level that follows the continuous Legendre shading function, with the drive levels gradually tapering from maximum at the center of the array to near-zero at the outer edges of the array (depending upon truncation of the arc formed by the curved speaker line array 100). The result is a speaker system that may provide wideband, extremely constant beamwidth and directivity behavior with virtually no side lobes.
A similar concept can be extended to a straight-line or flat-panel CBT arrays, with the use of appropriate signal delays. Such a technique is described, for example, in “Implementation of Straight-Line and Flat-Panel Constant Beamwidth Transducer (CBT) Loudspeaker Arrays Using Signal Delays,” 113th Convention of the Audio Engineering Society, Preprint 5653, October 2002, and “Full-Sphere Sound Field of Constant Beamwidth Transducer (CBT) Loudspeaker Line Arrays,” J. Audio Eng. Soc., vol. 51, no. 7/8, July/August 2003, both by D. B. Keele, Jr., and both of which are hereby incorporated by reference as if set forth fully herein.
Although CBT arrays have the potential for improved sound reproduction characteristics, they nonetheless still suffer from many of the same problems as conventional column array speakers. For example, the spacing between the driver units sets a limit to the upper frequency for directional control.
Another type of known speaker array comprises a two-dimensional array of forward-facing drive units. The two-dimensional array is composed of individual line arrays of drive units, with each line array offset or staggered from the neighboring line arrays. An example of such an array, which has been commercially marketed only recently (and therefore no admission concerning its potential status as prior art is intended by its inclusion in this background discussion of related art), is the YPS-1 “digital sound projector” available from Yamaha Electronics Corporation. The YPS-1 is described, for example, in a product brochure at http://www.yamaha.co.jp/english/product/av/pdfs/catalog/ysp1.pdf, as having 40 drive units of 4 cm size arranged in several rows, flanked at either end by a larger 11 cm drive unit. An on-board digital sound processor is apparently provided for controlling the drive units. The YPS-1 has various connectors including a coaxial video output to link it to a television, several digital inputs (optical and coaxial), and an RS-232C connector.
A two-dimensional array of forward-facing drive units, such as the YPS-1 or similar audio units, may potentially suffer from drawbacks such as lobing, and may also have limitations on the upper frequency response. Also, such a two-dimensional array may lack midrange warmth and body, and/or fail to convincingly reproduce certain audio sources, particularly music.
Accordingly, it would be advantageous to provide a speaker system which has a less adverse interaction between the loudspeaker and the listening room or area, and offers flexible directional control and/or steerability. It would further be advantageous to provide a speaker system that has more accurate sound timbre, and/or more accurate, believable and stable sound images over a wider listening area. It would also be advantageous to provide a speaker system that can be aesthetically packaged, and/or provides other benefits and advantages.